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Any method of negotiation may be fairly judged by three criteria: It should produce a wise agreement if agreement is possible. It should be efficient. And it should improve or at least not damage the relationship between the parties.
William Ury • Getting to Yes: Negotiating Agreement Without Giving In
when all we do in some cases is introduce people to the possible usefulness of the steering wheel...
Luke Rhinehart, Werner Erhard (Foreword), Joe Vitale (Introduction) • The Book of Est
Secrets of Power Negotiating for Salespeople: Inside Secrets from a Master Negotiator
amazon.com

any strategy development process must be consultative, one in which individual partners can become convinced that the benefits to them personally of achieving the goals will be worth the personal “costs” of doing new things or different things.
David H. Maister • Managing The Professional Service Firm

Book recommendation: Bargaining for Advantage: Negotiation Strategies for Reasonable People, by G. Richard Shell.